This invention relates to arrangements for the detection of failed fuel in nuclear reactors cooled by liquid metal.
The fuel for nuclear reactors of the fast breeder, liquid metal cooled kind is usually contained in closed sheaths in the form of pins, the pins being disposed side by side in axial disposition in an open-ended envelope usually termed a wrapper, there being a number of support and spacing grids by means of which the pins are retained and positioned in the wrapper, forming a fuel sub-assembly which, together with a multiplicity of other sub-assemblies in adjacent relationship form the core of the nuclear reactor, liquid metal being caused to flow upwardly through the sub-assembly to remove fission generated heat from the fuel pins by heat exchange. It is necessary to be able to detect any failure of the sheathing of the fuel pins of a particular sub-assembly so that such sub-assembly can be removed and replaced at the earliest opportunity to avoid contaminating the coolant and prejudicing the safety of the reactor.
Failed fuel detection for nuclear reactors of the kind described in the foregoing paragraphs has generally taken the form of employing one coolant sampling pipe for each fuel sub-assembly of the core. The pipes are terminated at respective ports of a multi-port selection valve by means of which sequential samples are taken, each being subjected to measurement for detection of the presence of fission products which would indicate one or more fuel sheathing failures in the particular fuel sub-assembly associated with the selected port. It is necessary to employ a liquid metal pump to effect circulation of each sample from the relevant sub-assembly to the selector, from thence to the measuring equipment, and from thence back to the liquid metal pool of the nuclear reactor. The kind of pump which has been found particularly effective for this purpose is a helical annular linear induction pump known in the art by the combination of its initials, HALIP. The HALIP can accommodate, in its annulus, a large number of ducts, to provide a variation of HALIP known in the art as a multi-duct HALIP. Reference is directed to British Patent Specification Nos. 1,413,304 and 1,556,258 for information concerning HALIPs.
Selector valves which rely on a contacting principle such as a rotary selection arm with a rubbing seal have the disadvantage that the rubbing seal can give trouble due to wear or foreign matter, and such failure prejudices the whole detection system. A solution to this problem is disclosed in European Published Patent Application No. 0084409 in which the rotary selector arm registers, in turn, with outlets of the sampling pipes but without making contact therewith and HALIPs are located upstream and downstream of the outlets whereby the upstream HALIP serves to impel the coolant across the gaps between outlets and selector arm and the downstream HALIP pumps the selected sample to a device for monitoring the selected sample for products indicative of fuel failure. The non-selected samples are collected following discharge from the sampling pipe outlets to provide a bulk sample which is fed to other failed fuel monitoring equiment. Such an arrangement requires three pumps, ie. the upstream and downstream HALIPs and a further pump for feeding the bulk sample to the bulk monitoring equipment and results in a complex construction involving at least two HALIPs, a mass of coolant sampling pipes and a rotary collector arm, as well as the necessary electrical cabling for the HALIPs, all of which must necessarily be accommodated as compactly as possible in view of the limited space available in the reactor for such instrumentation.